In the prior art, attempts have been made to improve the coupling between a single antenna and the input of a dual-band radio. In general, such prior art may be characterized as arrangements to isolate the antenna from the lead-in cable by an amplifier which provides high signal strength to the radio input regardless of losses in the lead-in line. Theoretically, good performance should be obtained by using a tuned amplifier with means to tune it to the tuned frequency of the radio; however, in practice, such arrangements are complicated and expensive and the expected results are not achieved. A less complicated arrangement uses an untuned amplifier which amplifies all signals in the band; however, it produces a greater increase in the noise output than in the signal output. Furthermore, an untuned amplifier produces mixing or cross modulation of stations in the received bands regardless of the selectivity of the receiver.
The Slavin U.S. Pat. No. 3,582,791 discloses an antenna coupling circuit in which an amplifier is connected to the base of an automobile antenna and a special coupling circuit is provided to eliminate the detuning effect of antenna length changes. The amplifier uses a field effect transistor (FET) having an input coupled to the antenna and an output inductively coupled to the special coupling circuit which includes a voltage dependent capacitor.
The Bannerman U.S. Pat. No. 4,001,696 discloses an antenna for automobile AM/FM receivers using a transistor preamplifier at the antenna. The Morrow U.S. Pat. No. 3,051,903 discloses an automobile radio, presumably an AM receiver, with a preamplifier at the antenna base.
In radio circuits, it is known to obtain impedance matching by use of particular amplifier configurations. For example, the use of a cathode follower power amplifier for coupling a high impedance source to a low impedance cable is described in "Radio Engineers Handbook", Terman, 1st Ed. (McGraw Hill, 1943), page 430.